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Significant and Subsequent Events - Clean Energy Investment Plan (Details) t in Millions, MWh in Millions, $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2020 CAD ($) project t MW | Dec. 31, 2020 CAD ($) project MWh t MW | Dec. 31, 2019 CAD ($) t | Dec. 31, 2018 CAD ($)
Investment [Line Items] |  |  |  |
Property, plant and equipment expenditures |  | $ 486 | $ 417 | $ 277
Assets | $ 9,747 | 9,747 | $ 9,508 |
Inventory write-down |  | $ 37 |  |
Coal inventory, number | t | 1.0 | 1.0 | 2.1 |
Coal inventory, increase due to mining | MWh |  | 2.3 |  |
Coal inventory, decrease due to consumption | t |  | 3.5 |  |
Depreciation and amortization |  | $ 654 | $ 590 | 574
Inventory write-down |  | $ 37 |  |
Gross carrying amount |  |  |  |
Investment [Line Items] |  |  |  |
Property, plant and equipment expenditures |  |  | $ 417 |
Sundance Unit 5 |  |  |  |
Investment [Line Items] |  |  |  |
Capacity of facility (in megawatts) | MW | 730 | 730 |  |
Sundance Unit 5 | Minimum |  |  |  |
Investment [Line Items] |  |  |  |
Payments for development project expenditure |  | $ 800 |  |
Sundance Unit 5 | Maximum |  |  |  |
Investment [Line Items] |  |  |  |
Payments for development project expenditure |  | $ 825 |  |
Keephills Unit 1 |  |  |  |
Investment [Line Items] |  |  |  |
Capacity of facility (in megawatts) | MW | 70 | 70 |  |
Sundance Unit 4 |  |  |  |
Investment [Line Items] |  |  |  |
Capacity of facility (in megawatts) | MW | 113 | 113 |  |
Clean Energy Investment Plan | ALBERTA |  |  |  |
Investment [Line Items] |  |  |  |
Number of wind projects | project | 1 | 1 |  |
Clean Energy Investment Plan | US |  |  |  |
Investment [Line Items] |  |  |  |
Number of wind projects | project | 3 | 3 |  |
Clean Energy Investment Plan | Maximum |  |  |  |
Investment [Line Items] |  |  |  |
Payments for development project expenditure |  | $ 35 |  |
Alberta Mine | Change in Useful Lives Estimate |  |  |  |
Investment [Line Items] |  |  |  |
Depreciation and amortization |  |  |  | $ 38
Alberta Mine | Mining assets |  |  |  |
Investment [Line Items] |  |  |  |
Inventory write-down | $ 37 |  |  |
Inventory write-down | 37 |  |  |
Alberta Mine | Mining assets | Change in Useful Lives Estimate |  |  |  |
Investment [Line Items] |  |  |  |
Depreciation and amortization | 15 |  |  |
Alberta Mine | Mining assets | Gross carrying amount |  |  |  |
Investment [Line Items] |  |  |  |
Assets | $ 373 | $ 373 |  |

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